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The Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group in southwest Japan

Nakada K., Matsuoka A.

Volumina Jurassica

2006

Vol. 4, no. 1

197-197

The Toyora Group is the continental shelf sediment exposed in the western part of Yamaguchi Prefecture, southwest Japan. It is mainly composed of sandstone and mudstone, and divided into three formations, that is, the Higashinagano, the Nishinakayama, and the Utano formations in ascending order. The Sakuraguchidani Mudstone Member, the lower part of the Nishinakayama Formation, is rich in ammonoids. Almost all ammonite specimens examined are classified into Hildocerataceae, and indicate successive ages from Upper Pliensbachian to Lower Toarcian. Hirano (1973) established three ammonite zones, in ascending order, the Fontanellense Zone, the Nipponicum Zone, and the Helianthoides Zone, and defined the Pliensbachian/Toarcian boundary within the P. nipponicum Zone. Furthermore, the black shale, a main component of the member, is known as a layer representing global Early Toarcian Ocean Anoxic Event (Tanabe 1991). The purpose of this study is to revise the Pliensbachian/Toarcian boundary in the Lower Jurassic Toyora Group by adopting the recently established global standard. Along the Sakuraguchidani route, the type locality of the Sakuraguchidani Mudstone Member, 99 ammonite specimens including 11 genera and 16 species were collected. The ammonite fauna is characterized by the abundance of Dactylioceras helianthoides (Yokoyama), Protogrammoceras nipponicum (Matsumoto), and Cleviceras chrysanthemum (Yokoyama). Paltarpites paltus (Buckman) and Fontanelliceras fontanellense (Gemmellaro) were obtained from this member. The Pliensbachian/Toarcian boundary is determined by adopting the definition which is used in Europe (e.g. Macchioni 2002). Among the various indexes, the first appearance of the genus Dactylioceras and P. paltus, mainly used in the Mediterranean area, is the most suitable for the Toyora Group. The boundary is drawn within the Fontanellense Zone, 23 m lower than that in the previous studies (Hirano 1973; Tanbe 1991). The Toyora Group has high potential for elucidating the timing of the Toarcian Ocean Anoxic Event off eastern Asia.